Overview

Pleural effusion is defined as the presence of excessive fluid in the pleural cavity resulting from transudation or exudation from the pleural surfaces. In normal conditions, the pleural space contains a small amount of fluid (≈0.3 mL·kg-1) maintained by a complex interplay of hydrostatic pressures and lymphatic drainage, which allows for steady liquid and protein turnover.[1] Pathological processes may lead to the development of pleural effusions by causing disequilibrium between the rates of pleural fluid formation, pleural permeability and pleural fluid absorption. Pleural effusion may be secondary to pleural processes, pulmonary disorders, systemic conditions, and medications. A systematic approach with a comprehensive clinical history and physical examination is required for establishing the etiology.

Pathophysiology

Healthy individuals have less than 15 ml of fluid in each pleural space. Normally, fluid enters the pleural space from the capillaries in theparietal pleura, from interstitial spaces of the lung via the visceral pleura, or from the peritoneal cavitythrough small holes in the diaphragm. This fluid is normally removed by lymphatics in the visceral pleura, which have the capacity to absorb 20 times more fluid than is normally formed. When this capacity is overwhelmed, either through excess formation or decreased lymphatic absorption, a pleural effusion develops.

Differentiating Pleural Effusion from other Diseases

Evaluation of a patient with a pleural effusion requires a thorough clinical history and physical examination in conjunction with pertinent laboratory tests and imaging studies. Thoracentesis should not be performed for bilateral effusions in a clinical setting strongly suggestive of a transudate unless there are atypical features or they fail to respond to therapy. Pleural fluid should always be sent for protein, lactate dehydrogenase, Gram stain, cytology and microbiological culture.[2] Additional studies which may be indicated in selected cases include pH, glucose, acid-fast bacilli and tuberculosis culture, triglycerdies, cholesterol, amylase, andhematocrit. Light's criteria is applied to distinguish the fluid between transudative or exudative.[3] A broad array of underlying conditions result in exudative effusions, while a limited number of disorders are assoicated with transudative effusions, which include congestive heart failure, cirrhosis, nephrotic syndrome, peritoneal dialysis, hypoalbuminemia, urinothorax, atelectasis, constrictive pericarditis, trapped lung, superior vena caval obstruction, and duropleural fistula.

Epidemiology and Demographics

In the United States, up to one million patients develop parapneumonic effusions annually, and approximately 100,000 patients undergo pleurodesis for recurrent pleural effusions per year.[4] Pleural effusion is reported to have an incidence of 0.32% in a study among the general population in central Bohemia. Congestive heart failure accounts for nearly 50% of cases, with malignancy, pneumonia and pulmonary emboli as the next three leading causes.[5] However, the distribution of causes is largely dependent on the population studied.

Diagnosis

Chest X Ray

Chest films acquired in the lateral decubitus position (with the patient lying on their side) are more sensitive, and can pick up as little as 50 ml of fluid. At least 300 ml of fluid must be present before upright chest films can pick up signs of pleural effusion (e.g., blunted costophrenic angles).